When Pluto transits natal Mercury, the intensity is mind-boggling. It’s like any other Pluto transit. You can’t glean what all is going on and probably won’t be able to assimilate it all for ten years!
You’re likely to experience a communication (Mercury) blackout (Pluto). This might happen because you shut down. In other cases, you may be shut out. What you say repulses people – they drop you.
You’ll have a lot of insight, though. Could be more than you’d like! Perceiving other people’s underpinnings isn’t necessarily pleasant.
There’s not a lot of relief with this transit. It can manifest as paranoia.
It’s also common to focus on something and read all about it. For example, a person could learn astrology during a Pluto transit to Mercury. You’re likely to be drawn to darker topics though, and may very well become obsessed.
What happened when Pluto squared your Mercury?